U.S. Patent Application Publication 2005/0068608 describes a process for selectively etching an etch layer in the form of a material layer. The process is used in the fabrication of microelectro-mechanical system (MEMS) devices. Features etched in the form of a via are fabricated in a layer structure of sub-micrometer thickness.
However, when etching thick layers with a thickness of more than 2 micrometers with a plasma etching process or a wet etching process, the control over critical lateral extensions at the bottom of the etch layer, i.e., at the interface between the etch layer and the substrate, is reduced. As a consequence, the mentioned critical lateral extension of the feature is observed to vary over a single wafer and also from wafer to wafer.
This problem can be very critical in the production of MEMS structures. For instance, the critical lateral extension may be a diameter of holes formed in a metal beam that is located above an electrode in a MEMS device. The metal beam can be moved to vary its distance from the electrode. In this example configuration, the capacity between the metal beam and the bottom electrode depends, among others, on the surface area on the bottom face of the metal beam, which faces the electrode. The surface area of the bottom face of the metal beam depends on the area of the holes that are provided as features with a critical lateral extension at the bottom face of the metal beam. Therefore, variations in the size of the holes at the bottom face of the metal beam are responsible for variations in the capacity of the MEMS device of this illustrative example.